Current insulated bearing components and bearings

ABSTRACT

Disclosed is a bearing component having at least one layer having a high hardness and a high current insulation property, the layer comprising a nonconductive oxide layer selected from the group comprising an Al 2 O 3  layer, a TaO layer, an SiO 2  layer, a mixed layer comprising two or more of the foregoing oxides, a multilayer structure comprising alternating layers of two or more of the foregoing oxides and a DLC layer such as a ta-C layer, there being at least one ALD layer comprising at least one layer of a material deposited by an ALD (atomic layer deposition) process on the at least one layer having a high hardness and a high current insulation property, the ALD layer itself having a high current insulation property and comprising a material or layer structure selected from the said group of materials.

The present invention relates to current insulated bearing componentsand bearings.

Rolling element bearings are used in many industrial applications, suchas in diverse machines, in wheel sets and traction motors of railvehicles, in DC and electric motors used in drive trains and ingenerators, such as those driven by wind power.

Such rolling bearings can be exposed to electrical current. In a worstcase scenario this can damage raceways and rolling elements, which inturn causes the motor or generator to fail prematurely and withoutwarning. On top of the extra expenses incurred for repairs, this alsomeans additional costs caused by machine downtime and the resultingproduction losses.

A much more economical solution is to provide for the use of currentinsulated bearings in the products concerned. This reduces themaintenance and repair costs and increases machine availability. Theseare all important issues for the customer.

In some cases it is sufficient to interrupt the electric circuit betweenthe bearing housing and the associated shaft and to install currentinsulated bearings at one or more bearing locations between the housingof the apparatus and the shaft, depending on the application.

Current insulated bearings, which are frequently ceramic coated, give asignificantly higher resistance to electrical current than standardbearings.

Generally speaking it is very difficult to eliminate the causes ofbearing voltages that are induced by the electric motor. I.e., moregenerally stated, to eliminate currents taking undesired paths, suchcurrents being related to the energy supply of the electrical deviceconcerned. Nevertheless, it is possible to avoid damage to the bearingif the flow of current can either be prevented or at least significantlyreduced. Many types of current insulated rolling bearings are availabletoday. The type of components which need to be insulated depends on thetype of voltages involved and the particular application orinstallation.

An induced voltage extending along the shaft of the rotor of theelectric motor or generator produces a circular current which is closedvia the bearings which support the shaft and the housing extendingbetween them. Such shaft voltages are often the result of anasymmetrical distribution of the magnetic flux within the motor. This isespecially evident in motors with only a few pairs of poles. In such acase it is sufficient to disrupt the flow of current by insulating oneof the two bearings. Situations also arise in which potentialdifferences exist between the shaft and the housing. In this caseelectrical currents flow through each of the bearings supporting theshaft in the same direction. The most likely cause of such potentialdifferences is the converters common-mode voltage. This type ofsituation might require the insulation of both bearings.

The type of electrical insulation which is to be used depends on thetime response of the relevant voltages. For DC voltages and lowfrequency AC voltages the ohmic resistance of the bearing is thedetermining property for current insulation. With higher frequency ACvoltages, which are often encountered in converters, the capacitivereactance of the bearing is an important parameter to consider whenselecting the current insulation property of the bearing. Basicallyspeaking, a current insulated bearing acts like a resistor and acapacitor connected in parallel. To ensure a good insulation, theresistance should be as high as possible and the capacitance should beas low as possible.

Irrespective of whether a bearing has been exposed to direct current oralternating current the resulting changes to the surface of the bearingare invariably the same, at least up to frequencies in the megahertzrange. In both cases the electrical current forms uniformly dull greymarks at the raceways on the rolling elements. These marks are not veryspecific and can also be caused by other factors (for example by a filmof lubricating oils containing abrasives). One can also find a type ofwashboard pattern that develops along the surface of the raceway of abearing extending in the direction of rotation. This type of damage,which is referred to as “fluting” indicates that an electrical currenthas passed through the bearing. If the damage found in bearings as aresult of current flow is examined under a scanning electron microscope,then it can be shown that the damage is characterized by densely packedcraters caused by local melting and welding beads with micron sizeddiameters covering the raceways. Such damage can be definitivelyaccepted as proof that electrical current has passed through thebearing. The craters and welding beads are the result of electricaldischarges between the microscopic peaks that are always found inraceways and on rolling element surfaces. When a spark penetrates afully developed lubricating film at a bottleneck, it causes the adjacentsurface to momentarily melt. In the mixed friction range (metal-to-metalcontact) the effective surfaces are temporarily fused together, thenimmediately broken apart again, by the rotation of the bearing. In bothcases material also separates from the surfaces where it immediatelysolidifies to form welding beads. Some of these beads mix with thelubricant, the remainder are deposited on the surfaces. Craters andwelding beads can be flattened and smoothed as the rolling elementscontinue to pass over them. If there is a continuous flow of current,the usually thin surface layers repeat the melting and solidifyingprocess over and over again in the course of time.

Most actual bearing failures result from the “fluting” referred to abovewhich seems to originate as a result of the combined effect of thecontinuous flow of electrical current and the vibrational properties ofthe bearing components. Each time the rolling element comes into contactwith a sufficiently large crater it becomes radially displaced; theextent of the element's displacement depends on the bearing's internalgeometry and speed as well as on the loads acting on the bearing. As therolling element swings back the thickness of the lubricating film iseroded, resulting in more sparkovers in this area. A self-sustainingprocess has been triggered. After a while the entire circumference ofthe ring's raceway can be covered by fluting damage. This leads to morepronounced bearing vibrations, finally leading to bearing failure. Areliable criterion for assessing the level of danger posed by electricalcurrent is referred to in the art as the “calculated current density”,that is to say the effective amperage divided by the total area ofcontact between the rolling elements and the bearing's inner ring andouter ring. The current density is dependent on the type of bearing andon the operating conditions. According to current experience there isnormally no risk of fluting when current densities are less thanapproximately 0.1 A_(eff)/mm². Densities that are above 1 A_(eff)/mm²are, however, likely to cause this type of damage.

The electrical current also negatively affects the lubricant. The baseoil and additives in the oil tend to oxidize and cracks develop. This isclearly evident under the infrared spectrum. The lubricating propertiesare compromised by premature aging as well as by increased concentrationof iron particles which can cause the bearing to overheat.

Having regard to the need to provide current insulated bearings, thetypical concept is to use plasma spraying to apply oxide ceramiccoatings. A special sealant helps the sprayed ceramic coating to retainits insulating properties even in a damp environment. The resultingoxide ceramic coating is very hard, wear resistant, and a good thermalconductor. Sometimes the outer race is coated at the outer side andsometimes the inner race is coated at the inner side. The bearings areusually made in such a way that the thickness of the coating is takeninto account so that the ceramic coated bearings are interchangeablewith standard bearings, for example in accordance with DIN 616 (ISO15).Bearings can be deep groove ball bearings and are available in both openand sealed versions (with lip seals on one or both sides). This enablesthe user to also benefit from the advantages offered by lubrication forlife.

The plasma spraying process involves the generation of an arc betweentwo electrodes to ionize a noble gas that is issued from the plasmatorch. The resulting plasma jet is used to carry the injected aluminiumoxide powder which is melted by the heat and sprayed at high speed ontothe outer or inner ring. When applied in this manner, the oxide layeradheres extremely well to the base material and is then seated andground to size. Current coatings are available that guarantee adielectric strength of at least 1000 VDC or of at least 500 VDC.

Below this voltage the insulating layer only permits extremely lowlevels of current flow through the bearing. It offers resistance to DCcurrents and AC currents.

At room temperature the sprayed ceramic layer typically has a DCresistance of 1 to 10 GOhm depending on the bearing size. As thetemperature increases the DC resistance decreases exponentially,typically by approximately 40 to 50% per 10° K. However, even atoperating temperatures of 60° C. or even 80° C., the insulting layerstill has a resistance of several MOhm. According to Ohm's law (i.e. Iequals V divided by R) this means that voltages up to 1000 V onlyproduce currents that are significantly below 1 milliampere which arenot critical for bearings.

Having regard to AC resistance the insulating unit acts like a capacitorwhich can accumulate charges. When exposed to an AC voltage, this causesan alternating current to flow through the contact area between therolling element and the raceways. In the case of a harmonic timedependence with an angular frequency w, the root mean square values forcurrent and voltage are calculated using the formula I=V.ω.C.

Analogous to Ohm's law Z=1/ωC is the capacitive reactance of thebearing. A bearing with an oxide ceramic coating typically has acapacitance of 2 to 20 of depending on the bearing size. Thus, at afrequency of 50 Hz, it has a capacitive reactance of 0.15 to 1.5 MOhmwhich is significantly lower than its DC resistance. At higherfrequencies this value decreases even further. Nevertheless, in mostcases, it will be significantly higher than the resistance of anon-insulated bearing which, at voltages higher than 1 V is very low (1Ohm and less). The coating thicknesses that are used vary from somewhatless than 100 μm to average values of 200 μm or even above 200 μm.

One particular condition which should also be observed in the prior artis that the surfaces to be coated must be cylindrical, they must not beinterrupted by lubricating holes or grooves.

Also known are hybrid bearings which typically have rings made fromrolling bearing steel but ceramic rolling elements. The rolling elementsare essentially wear-free and provide the requisite current insulation.Such bearings have a greater resistance to the passage of current thanceramic coated bearings. Even at high temperatures the DC resistance isin the GOhm range. The bearings typically have a capacitance of about 40pf which is lower than for ceramic coated bearings by a factor of 100.Such rolling bearings have lower friction at higher speed which meansreduced operating temperatures. They also have better dry runningproperties. Such hybrid bearings typically also have a longer greaselife than traditional lubricated for life bearings.

Although the above description of the prior art relates generally torolling element bearings, similar problems can arise in linear bearingsand sliding bearings so that it can be advantageous in somecircumstances to provide current insulation for such linear bearings orsliding bearings.

The principal object underlying the present invention is to provide abearing component having a high hardness and high current insulationproperties, the insulation property being understood to mean a voltageinsulation property better than 500 VDC and preferably better than 1000VDC, for an effective current of less than 0.1 A_(e)ff/mm² with thesevalues being achieved for significantly thinner coatings than thepreviously known coatings.

Furthermore, it is an object of the present invention to provide currentinsulated bearings in which grooves or lubrication holes in races are nolonger a problem with regard to current insulation.

An additional object of the invention is to provide a coating which canbe used not only on bearing races, but also on cages for the rollingelements and on the rolling elements themselves.

It is a yet further object of the present invention to provide currentinsulating coatings with high hardness which are relatively inexpensiveto apply and which have excellent current insulation properties forvarious layer thicknesses. It is also desired to produce a currentinsulating coating having high uniformity and good reproducibility.

In order to satisfy this object there is provided, in accordance withthe present invention, a bearing component for a linear bearing of arolling element bearing and a sliding bearing, for example a componentselected from the group comprising a bearing race, a rolling elementsuch as a tapered roller, a barrel roller, a needle roller, a bearingball and a rolling element cage, the bearing component having at leastone layer having a high hardness and a high current insulation propertyapplied by a PVD (physical vapor deposition) process, by a CVD (chemicalvapor deposition) process, or by a PECVD (plasma enhanced chemical vapordeposition) process (but excluding an ALD process or a plasma enhancedALD process) at at least one surface region of said article, said atleast one layer comprising a non-conductive oxide layer selected fromthe group comprising an Al₂O₃ layer, a TaO layer (more generally aTa_(x)O_(y) layer), an SiO2 layer (more generally a Si_(x)O_(y) layer),a mixed layer comprising two or more of the foregoing oxides, amultilayer structure comprising alternating layers of two or more of theforegoing oxides and a DLC layer such as a ta-C layer, there being atleast one ALD layer comprising at least one layer of a materialdeposited by an ALD (atomic layer deposition) process on said at leastone layer having a high hardness and a high current insulation property,the ALD layer itself having a high current insulation property andcomprising a material or layer structure selected from the said group ofmaterials.

The invention will now be explained in more detail with reference to theaccompanying drawings in which are shown:

FIG. 1 a schematic view of a cathode sputtering apparatus for depositingDLC coatings,

FIG. 2 a cross-section through a modified version of the vacuum chamberof the apparatus of FIG. 1,

FIGS. 3A-3C three sequential steps of an example for the deposition ofan ALD layer,

FIG. 4A a first composite coating in accordance with the presentinvention,

FIG. 4B an enlarged view of a section of the composite coating of FIG.4A,

FIG. 4C an enlarged view of the coating of FIGS. 4A and 4B after thesurface has worn away,

FIG. 4D an enlarged view similar to FIG. 4B but with a thinner ALDsealing layer,

FIG. 4E an enlarged view of the coating of FIG. 4D after the surface hasworn away,

FIG. 5 a chamber used for depositing ALD coatings and

FIGS. 6A-6F examples of coating systems which are used for embodimentsof the present invention.

In all drawings the same reference numerals have been used for the samecomponents or features or for components having the same function andthe description given for any particular component will not be repeatedunnecessarily unless there is some distinction of importance. Thus adescription given once for a particular component or feature will applyto any other component given the same reference numeral. Also theinvention will be understood to include a method of coating a bearingcomponent using PVD, CVD, PECVD processes for a first layer and ALD fordepositing a second layer on the first layer.

By way of an introduction to DLC coatings (Diamond Like CarbonCoatings), reference can be made to a paper entitled “Diamond-likeCarbon Coatings for tribological applications on Automotive Components”by R. Tietema, D. Doerwald, R. Jacobs and T. Krug presented at the 4thWorld Tribology Congress, Kyoto, September 2009. That paper discussesthe manufacture of diamond-like carbon coatings from the beginning ofthe 1990's. As described there, the first diamond-like carbon coatings(DLC-coatings) were introduced on the market for automotive components.These coatings enabled the development of HP diesel fuel injectiontechnology.

The German standard VDI 2840 (“Carbon films: Basic knowledge, film typesand properties”) provides a well-defined overview of the plurality ofcarbon films, which are all indicated as diamond or diamond-likecoatings.

The important coatings for tribological applications are thehydrogen-free tetragonal “ta-C” coatings and further coatings of thiskind with incorporated hydrogen referred to as ta-C:H coatings. Also ofimportance for tribological applications are amorphous carbon coatingswith or without incorporated hydrogen which are respectively referred toas a-C coatings and a-C:H coatings. Moreover, use is frequently made ofa-C:H:Me coatings which include metal carbide material such as tungstencarbide. a-C:H coatings can be deposited in known manner by CVD andespecially by plasma enhanced CVD processes and by PVD processes. PVDprocesses are also used to deposit and a-C;H:Me coatings. Theseprocesses are well known per se as shown by the paper referred to aboveand they will not be described further here.

To date, the ta-C coatings have been made using an arc process. Ahardness in the range from 20 GPa to 90 GPa, in particular 30 GPa to 80GPa; is considered useful (diamond has a hardness of 100 Gpa). However,the coatings are quite rough because the arc process leads to thegeneration of macroparticles. The surface has rough points due to themacroparticles. Thus although low friction can be obtained the wear rateof the counterpart in the tribological system is relatively high due tothe surface roughness caused by macroparticles.

Hydrogen free ta-C coatings are of particular interest for the presentinvention because of their good electrical insulating properties.

Referring first to FIG. 1 a vacuum coating apparatus 10 is shown forcoating a plurality of substrates or workpieces 12. The apparatusincludes a vacuum chamber 14 of metal, which in this example has atleast one, preferably two or more magnetron cathodes 16 which are eachprovided with a high power impulse power supply 18 (of which only one isshown here) for the purpose of generating ions of a material which ispresent in the gas phase in the chamber 14, i.e. inert gas ions and/orions of the materials of which the respective cathodes are formed. Twoof the cathodes 16 are preferably oppositely disposed for operation in adual magnetron sputtering mode. This can be advantageous for thedeposition of Al2O3 coatings by magnetron sputtering as will bedescribed later in more detail. The workpieces 12 are mounted on holdingdevices on a support device in the form of a table 20 which rotates inthe direction of the arrow 22 by means of an electric motor 24. Theelectric motor drives a shaft 26 which is connected to the table 20. Theshaft 26 passes through a lead-through 28 at the base of the chamber 14in a sealed and isolated manner which is well known per se. This permitsone terminal 30 of the bias power supply 32 to be connected via a lead27 to the workpiece support table 20 and thus to the workpieces. Thissubstrate bias power supply 32 is shown here with the letters BPS, anabbreviation for bias power supply. The BPS is preferably equipped withHIPIMS-biasing capability, as described in the EP application 07724122.2published as WO2007/115819, in particular with regard to the embodimentof FIGS. 1 to 3 of that document. Although only a single rotation isshown here for the table 20, the trees 29 of the holding devices for theworkpieces 12 can also be rotated about their own longitudinal axes(two-fold rotation) and if desired the workpieces can be rotated abouttheir own axes (three-fold rotation) if the holding devices areappropriately designed.

Biasing can also be done by pulsed biasing or RF-biasing. Pulsed biasingcan be synchronized with the HIPIMS-cathode pulses (also described inWO2007/115819). Good results can be achieved with the HIPIMS-DC biasingdescribed in connection with FIGS. 1 to 3 of WO2007/115819.

In this embodiment the metallic housing of the vacuum chamber 14 isconnected to ground. The positive terminal(s) of the high impulsecathode power supply(ies) 18 is/are likewise connected to the housing 14and thus to ground 36 as well as the positive terminal of the bias powersupply 32.

A further electric voltage supply 17 is provided for use when theapparatus is operated in a plasma enhanced chemical vapor depositionmode (PECVD) and will be explained later in more detail. It can beconnected to the rotary table 20 instead of the bias power supply 32 byway of the switch 19. The electric voltage supply 17 is adapted to applya periodically variable medium frequency voltage in the range of betweenof up to 9,000 volts, typically 500 to 2,500 volts, and at a frequencyin the range between 20 and 250 kHz to the workpieces 12 mounted on thetable 20.

A connection stub 40 is provided at the top of the vacuum chamber 14(but could be located at other locations as well) and can be connectedvia a valve 42 and a further duct 44 to a vacuum system for the purposeof evacuating the treatment chamber 14. In practice this connection stub40 is much larger than is shown, it forms the connection to a pumpingstand which is suitable for generating a high vacuum in the chamber andis flanged onto the duct 44 or directly onto the chamber 14. The vacuumsystem or pumping stand is not shown but well known in this field.

A line 50, which serves for the supply of an inert gas, especially argonto the vacuum chamber 14, is likewise connected to the top of the vacuumchamber 14 via a valve 48 and a further connection stub 46. Forsupplying other process gases such as acetylene, oxygen or nitrogen,additional gas supply systems 43, 45, 47 can be used.

Vacuum coating apparatuses of the generally described kind are known inthe prior art and frequently equipped with two or more cathodes 16. Forexample a vacuum coating apparatus is available from the company HauzerTechno Coating BV in which the chamber has a generally square shape incross-section with one cathode at each of the four sides. This designhas one side designed as a door permitting access to the chamber 14. Inanother design the chamber is approximately octagonal in cross-sectionwith two doors which each form three sides of the chamber. Each door cancarry up to three magnetrons and associated cathodes 16. A typicalvacuum coating apparatus includes a plurality of further devices whichare not shown in the schematic drawings of this application. Suchfurther devices comprise items such as dark space shields, heaters forthe preheating of the substrates and sometimes electron beam sources orplasma sources in diverse designs. An ion source for use in the plasmaenhanced chemical vapor deposition mode is shown in FIG. 1 by thereference numeral 21 and is positioned generally on the centrallongitudinal axis of the vacuum chamber. It can be a resistance heatingfilament connected to its own power supply or any other known design ofion source. The ion source 21 is connected to the negative output of adirect voltage supply (not shown). The positive pole of the directvoltage supply can be applied by way of a switch to the table 20 andthus to the holding devices and the workpieces 12 during the PECVDcoating process.

The vacuum chamber of FIG. 1 is also equipped with two coils 23 and 25at the top and at the bottom of the chamber respectively. These can beconnected to a DC power supply or to respective DC power supplies, theyoperate as Helmholz coils and enhance the magnetic field along the axisof the chamber. The current flows through each of the coils 23 and 25 inthe same sense. It is known that the plasma intensity and the currentflowing at the workpieces 12 are proportional to the current flowing inthe coils 23 and 25 and thus to the magnetic field generated thereby.

It is also possible to provide arc cathodes with respective arc powersupplies in the same chamber in addition to magnetron cathodes.

The individual items of the coating apparatus are preferably allconnected to a computer based process control. This makes it possible tocoordinate all the basic functions of a vacuum coating apparatus (vacuumpumping system, vacuum level (pressure in the vacuum chamber), powersupplies, switches, process gas supplies and gas flow control, currentsin the coils 23 and 25, positions of any variably positioned magnets,safety controls etc.). It also makes it possible to allow the specificvalues of all relevant variable parameters to be flexibly matched at anypoint in time to the coating or process requirements and to producecoatings to specific repeatable recipes.

When using the apparatus air is first extracted from the vacuum chamber14 by the vacuum pumping system via the duct 44, the valve 42 and thestub 40 and argon is supplied via the line 50, the valve 48 and theconnection stub 46. The chamber and the workpieces are preheated duringpump-down to drive out any volatile gases or compounds which adhere tothe workpieces or chamber walls

The inert gas (argon), which is supplied to the chamber, is alwaysionized to an initial extent, for example by cosmic radiation and splitsup into ions and electrons.

By generating a sufficiently high negative bias voltage on theworkpieces, a glow discharge can be generated on the workpieces. Theargon ions are attracted to the workpieces and collide there with thematerial of the workpieces, thus etching the workpieces.

Alternatively, Ar ions can be generated by a plasma source. Thegenerated ions are attracted to the workpieces 12 by the negativesubstrate bias voltage and etch the workpieces 12.

As soon as the etching treatment has been carried out, the coating modecan be switched on. For a sputter discharge the cathodes will beactivated during deposition. Ar ions collide with the target and knockatoms out of the target. Electrons are ejected from the target due tosputtering and are accelerated by the dark space voltage gradient. Withtheir energy they can collide with Ar atoms, where secondary electronswill be emitted and help to maintain the discharge. Each of the cathodesis provided with a magnet system (not shown in FIG. 1) which is wellknown per se and which normally generates a magnetic tunnel in the formof a closed loop which extends over the surface of the associatedcathode. This tunnel formed as a closed loop forces the electrons tomove around the loop and collide with argon atoms causing furtherionization in the gas atmosphere of the vacuum chamber 14. This in turncauses further ionization in the chamber from the material of theassociated cathode and the generation of further argon ions. Duringdeposition these ions can be attracted to the substrates by the appliednegative bias voltage of for example 10 V to 1200 V and strike thesurface of the workpieces with appropriate energy to control the coatingproperties.

In case of a HIPIMS discharge, a different discharge mode is effective.The number of ions increases dramatically and as a consequence thetarget material particles knocked out from the target will be ionized.This is not the case for a normal sputter discharge. As a consequencegases present in the chamber will be highly ionized as well. This isparticularly beneficial when dopants are applied.

The power supply to the cathode or cathodes causes a flux of ions of thematerial of the cathode to move into the space occupied by theworkpieces 12 and to coat them with the material of the respectivecathode. The structure of the coating is influenced by the appliednegative bias voltage that influences the movement of ions towards theworkpieces.

Sputtering processes are known in diverse forms. There are those thatoperate with a constant voltage at the cathodes and a constant negativevoltage at the workpieces and this is termed DC magnetron sputtering.Pulsed DC sputtering is likewise known in which at least one of thecathodes is operated in a pulsed mode, i.e. pulsed power is applied tothe cathode by a pulsed power supply.

A special form of a pulsed discharge is the HIPIMS discharge. In aHIPIMS mode the power which is supplied to each cathode during a powerimpulse can be much higher than the power of a DC sputtering modebecause there are substantial intervals between each pulse. However, theaverage power remains the same as for DC puttering. The limitingconstraint on the power is the amount of heat that can be dissipated atthe cathode before this overheats.

The use of HIPIMS leads to a higher ionization in the vacuum chamber andimproved coatings. For example, in well-known HIPIMS sputtering (highpower impulse magnetron sputtering), each power pulse can have aduration of say 10 μs and a pulse repetition time is used of say 2000μs, (corresponding to a pulse repetition frequency of 500 Hz, i.e. aspacing between impulses of 1990 μs). As another example, the pulserepetition frequency might be 50 Hz and the pulse duration 100 μs, i.e.a spacing between impulses of 20 ms-100 μs. These values are only givenas an example and can be varied in wide limits. For example, an impulseduration can be selected between 10 μs and 4 ms and a pulse repetitiontime between 200 μs and 1 s. As the time during which a very high peakpower is applied to the cathodes is short, the average power can be keptto a moderate level equivalent to that of a DC sputtering process. Ithas been found that by the application of high power impulses at thecathode these operate in a different mode in which a very high degree ofionization of the ions arises which are ejected from the cathodes: Thisdegree of ionization, which is material dependent, can lie in the rangebetween 40% and 90%. As a result of this high degree of ionization, manymore ions are attracted by the workpieces and arrive there with highervelocities which lead to denser coatings and make it possible to achievecompletely different and better coating properties than is possible withregular sputtering or arc coating.

The fact that the power is supplied in power peaks means, however, thatrelatively high currents flow in the bias power supply during thesepower peaks and the current take up cannot be readily supplied by anormal power supply.

In order to overcome this difficulty, WO 2007/115819 describes asolution as shown in FIG. 1 of this application in connection with thebias power supply BPS (32) in which an additional voltage source 60 isprovided. The additional voltage source 60 is best realized by acapacitor. The capacitor 60 is charged by a customary bias power supplyto the desired output voltage. When a power impulse arrives at one ofthe cathodes from the HIPIMS power supply 18 then this leads to anincreased material flow of ions, essentially ions of the cathodematerial to the workpieces 12 and this signifies an increase of the biascurrent at the bias power supply via the workpiece support table 20 andthe line 27. A normal bias power supply could not deliver such a peakcurrent when it is designed for constant DC operation instead of HIPIMSoperation. However, the capacitor 62, which is charged by the bias powersupply to the desired voltage in the periods between the power impulses,is able to keep the desired bias at the substrates constant withinnarrow limits and to supply the required current which only causes asmall degree of discharging of the capacitor. In this way, the biasvoltage remains at least substantially constant.

By way of example the discharge can take place in such a way that a biasvoltage of −50 V drops during the power pulses to −40 V.

In a simple form of the present teaching one of the cathodes 16 is a Cr,Ti or Si target for supplying a bond layer material. Possibly, othermaterials could also be used for a bond layer.

When depositing a DLC layer in the form of a ta-C layer the workpieceswere positioned on a table 20 and were made by a PVD arc process from acarbon cathode in manner known per se. The chamber 10 had a workingheight of the space in which the workpieces are located of 850 mm. Toensure good adhesion of the hard hydrogen-free carbon layer on thesubstrate, the apparatus initially used a standard ARC adhesion layersuch as is used when depositing ta-C by carbon arc. It will not bedescribed in detail because it is not the preferred solution and the arcprocess is in any case well known.

FIG. 2 shows a view of the vacuum chamber of FIG. 1 in a cross-sectionperpendicular to the vertical axis with additional detail but withoutthe workpieces. The chamber also has four cathodes, one of Cr as a bondlayer material, one of graphite as a source of carbon and two ofaluminium for forming an Al2O3 layer by dual magnetron sputtering in areactive oxygen atmosphere.

The two cathodes 16 labelled also Al are of aluminium and have magnetarrangements with center poles of polarity “north” (N) and outside polesof polarity “south” (S) to generate the well-known magnetic tunnel of amagnetron. The cathodes have the shape of elongate rectangles whenviewed face on and are shown here in a cross-section perpendicular totheir long axis. Instead of having SNS polarity as shown, they couldhave NSN polarity as shown for the magnet arrangements for the cathodesof Cr and C at the top and bottom of FIG. 2. The cathodes 16 of Cr and Cwould then have magnet arrangements with SNS polarity.

The magnet arrangements can be moved in the direction of the respectivedouble arrows 82 towards and away from the respective cathodes 16. Thisis an important control parameter for the operation of the HIPIMScathodes.

The idea is for the magnetrons to have alternating polarities goingaround the vacuum chamber 14. This means, with an even number ofcathodes, that the magnetic poles always alternate, i.e. N, S, N, S, N,S, N, S, N, S, N, S, when going around the chamber. This leads to anenhanced magnetic confinement of the plasma. A similar magneticconfinement can also be achieved if all cathodes have the samepolarities, say NSN. Then it is necessary to operate with auxiliary Spoles between the adjacent magnetrons to obtain a similar N, S, N, S, Narrangement around the chamber. It will be appreciated that thedescribed arrangements only work with an even number of magnetrons.However, it is also possible to obtain a similar effect with an oddnumber of magnetrons either by making some poles to be stronger thanothers or by the use of auxiliary poles. Such designs to obtain a closedplasma are well known and documented in various patent applications. Itis not essential that a closed plasma is achieved.

What FIG. 2 also shows is four rectangular coils 80 positioned outsideof the chamber 14 like the magnets with the SNS poles or NSN poles. Thecoils form electromagnets and have the same polarity as the outermagnets for the respective cathodes 16. These electromagnetic coils 80enable the magnetic flux in front of the cathodes 16 and inside thechamber 14, to be varied.

The vacuum coating system can be operated as follows:

The chamber and the workpieces located in it are first evacuated to alow pressure less than 10⁻⁴ mbar, such as 10⁻⁵ mbar, and preheated whilesupplying argon to the chamber at a flow rate of, for example 75 sccm.During this period the heating of the chamber and the workpieces drivesout contamination such as gases and water adsorbed on the surface of theworkpieces and on the chamber walls and this contamination is removed bythe vacuum system along with the remaining environmental gas in thevacuum chamber and a proportion of the argon gas that is supplied. Thusthe argon gas gradually flushes the vacuum chamber. After thispreheating and cleaning step further cleaning is effected during acleaning and etching treatment. This treatment is carried out on theworkpieces 12 with Ar ions, using the argon atmosphere in the vacuumchamber. This step is carried out for a period of 10 to 30 minutes. Theion source can be the ion source 21 referred to above or another ionsource.

Another option for the etching step is the use of HIPIMS etching with aCr, Ti or Si target operated in a HIPIMS magnetron etching mode with arelatively high substrate bias of −500 to −2000 V. This is well known inthe art and described in EP-B-1260603 of Sheffield Hallam University.The typical time averaged equivalent DC etching power applied to the Cr,Ti or Si cathode is in the range of 1 to 25 kW.

In a second step a bond layer of Cr, Ti or Si is deposited on the metalsurface. This is done for about 10 to 20 minutes from a target of Cr, Tior Si operated in a sputter discharge mode or in a HIPIMS coating mode.In this connection it should be noted that, in case of using a HIPIMSmode, the maximum average power which can be dissipated by and thuseffectively applied to a cathode is the power which does not lead to anundesirable temperature increase of the cathode or unwanted meltingthereof. Thus in a DC sputtering operation a maximum power ofapproximately 15 W/cm2 might be applied to a particular cathode in thecase of indirectly cooled targets, corresponding to the allowablethermal load of the target. In HIPIMS operation a pulsed power supply isused which might typically apply power in 10 to 4000 μs wide pulses at apulse repetition frequency of less than 1 Hz-5 kHz. In an example: ifthe pulse is switched on during 20 μsec and a pulse frequency of 5 kHzis applied, each pulse would then have a power associated with it of 180kW resulting in an average power of

P=180 kW×(20 μs/(200−20)μs=20 kW

For this example the maximum pulse power that can be supplied during aHIPIMS pulse is thus 180 kW.

An appropriate negative substrate bias of about 0 to 200V should beprovided during the deposition of the bond layer. The pressure in thechamber may be between 10⁻and 10⁻³ mbar. The deposition of the bondlayer can also be done with filtered arc cathodes. Also the use ofunfiltered arc cathodes is a possibility, but this is less advantageousbecause it will lead to additional roughness of the coating because ofdroplet generation.

In a third step a Cr—C, Ti—C or Si—C transition layer is deposited forabout 1 to 5 minutes with simultaneous operation of a Cr, Ti or Sitarget and a graphite target in a HIPIMS mode or with carbon-arccathodes with about −50 to −2000 V substrate bias. The pressure in thechamber can again be in the range between 10⁻⁴ and 10⁻³ mbar.

Thus, the apparatus of the invention typically comprises a plurality ofmagnetrons and associated cathodes, at least one of which comprises abond layer material (Cr, Ti or Si). The at least one cathode for thebond layer material can also be an arc cathode (filtered, orunfiltered). The apparatus further comprises a power supply for thesputtering of bond layer material for the deposition of the bond layermaterial on the substrate or substrates prior to deposition of the DLClayer. A typical example of a bond layer material is as already statedCr, Ti or Si. Thus there will usually be a minimum of two cathodes,typically one of Cr and one of graphite. In practice it may be moreconvenient to use a sputtering apparatus with four or more cathodes.This makes it relatively easy to arrange the magnetrons and/or the arccathodes so that there is an alternating pole arrangement of N, S, N;(magnetron 1) S, N, S (magnetron 2); N, S, N (magnetron 3) and S, N, S(magnetron 4) arranged around the periphery of the vacuum chamber inmanner known per se to ensure stronger magnetic confinement of theplasma (closed field).

The pulse repetition frequency is preferably in the range from 1 Hz to 2kHz, especially in the range from 1 Hz to 1.5 kHz and in particular ofabout 10 to 30 Hz.

If an a-C:H or a ta-C coating is used then dopants can be added to thecoating. In this respect dopants can be metals from sputter targetsoperated with arc sputtering or magnetron sputtering or from HIPIMScathodes (Si, Cr, Ti, W, WC). The dopants can also be supplied fromprecursors in gas phase (such as hydrocarbon gases, nitrogen, oxygen, Sicontaining precursors like silane, HMDSO, TMS). The invention alsocomprises the use of dopants to the ta-C coating) so long as the dopantsdo not undesirably reduce the insulating properties so that thecalculated current density exceeds the permissible value.

A specific example for the deposition of an a-C:H layer, i.e. a DLClayer coating containing hydrogen will now be described:

In known manner the treatment process is first started by pumping thechamber down to a relatively low pressure at least one order ofmagnitude below the actual chamber pressure for the deposition process,e.g. to a pressure less than 10 ⁴ such as 10 ⁵ mbar. During this, orfollowing this, the chamber and its contents are subjected in knownmanner to a heating process to drive of the volatile gases in thechamber and adsorbed onto the surfaces of the chamber and the itemspresent there. During the preheating a flow of argon is maintained inthe chamber by supplying argon through the inlet and removing it via thevacuum pump. The heating phase typically lasts about 20 to 25 minutes.

Following the preheating process etching takes place once a steadytemperature has been reached. The etching is for example carried outusing a HIPIMS etching process as covered by European patentEP-B-1260603 although other etching processes can also be used. Duringetching argon gas is supplied to the vacuum chamber for example at 75sccm and ionized by operation of one or more of the magnetronsincorporated therein, for example the magnetron with the target 16 of Crcan be used.

If deemed necessary, the workpiece can be provided with an adhesivelayer, also termed a bond layer, to facilitate the adhesion of the DLCcoating. Such a bond layer is not always necessary. For some workpiecematerials, especially those with a content of Cr, Ti or Si, such as 100Cr6, the DLC layer, or some types of DLC layer could be depositeddirectly on the cleaned and etched workpieces without the use of a bondlayer. If an adhesive layer is provided on the workpiece then it can beselected from the group of elements of the IV, V and VI Subgroup as wellas Si. Preferably an adhesive layer of the elements Cr or Ti is usedwhich have been found to be particularly suitable for this purpose.

The adhesive layer can be deposited by arc sputtering or filtered arcsputtering, but is preferably deposited using magnetron sputtering fromthe Cr target 16 in FIG. 2.

Again, argon is supplied to the vacuum chamber. During this phase theargon flow is higher than during pre-heating and etching and may, forexample, be set at 120 sccm. The pressure in the vacuum chamber istypically around 10⁻³ mbar but can be up to an order of magnitude loweror can be somewhat higher than 10⁻³ mbar. A negative bias of around 50 Vis applied to the substrate carrier and the deposition of the bondinglayer takes only a few minutes with about 10 kW of power applied to thecathode (average power if the magnetron cathode is operated in theHIPIMS mode).

It can also be advantageous to provide a gradient layer between theadhesive layer and the DLC layer. Such a gradient layer can furtherimprove the adhesion of the DLC layer on the workpiece.

The idea of the gradient layer is to progressively reduce the proportionof Cr in the gradient layer while increasing the proportion of carbon init thus forming chromium carbide and allowing the carbon content toincrease until only a DLC coating is being applied.

There are several possibilities for depositing the gradient layer. Onepossibility is to operate the magnetron with the carbon target 16simultaneously to the Cr target, e.g. again using HIPIMS sputtering. Thepower supplied to the Cr target is progressively reduced, or reduced insteps, while the power supplied to the C target is progressivelyincreased, or increased in steps. Another possibility is to add carbonto the vacuum chamber in the form of a reactive gas such as acetylene ormethane and to progressively increase the amount of carbon present inthe atmosphere of the chamber while reducing the power supplied to theCr target.

Another possibility is to use the technique described in EP-B-1272683for the deposition of an adhesive layer, of a graded layer andsubsequently a DLC layer.

If that process is used then, after deposition of part of the Cr layer,i.e. part of the adhesive layer, the substrate bias is switched overfrom direct current to medium frequency by using the switch 19 toconnect the electric voltage supply 17, which is a bipolar generator, tothe table 20 instead of the constant bias supply 32. The electricvoltage supply is operated with a preferred amplitude voltage of between500 and 2,500 V, for example 700 V, and a frequency between 20 and 250kHz, for example 50 kHz. The pressure in the vacuum chamber is typicallyaround 10⁻³ mbar, but can be up to an order of magnitude lower or can besomewhat higher than 10⁻³ mbar. After approximately 2 minutes, anacetylene ramp is started at 50 sccm and is raised over a time period ofapproximately 30 minutes to 350 sccm. Approximately 5 minutes afterswitching on the medium frequency generator, the power of the used Crtarget is reduced to 7 kW; after another 10 minutes, it is reduced to 5kW and is held constant there for another 2 minutes. Thus, for thegeneration of a graded adhesive layer acetylene (or another carboncontaining gas) can be supplied to the vacuum chamber in increasingamounts during the deposition of the adhesive or bonding layer afterabout one third of that layer has been deposited so that the compositionof the adhesive layer or bonding layer progressively changes fromchromium to chromium carbide.

Once the gradient layer has been completed, screens are moved in frontof the targets and these are switched off, whereby the depositing of the“pure” DLC layer starts which is constructed essentially of carbonatoms, of low quantities of hydrogen and of still lower quantities ofargon atoms.

For this purpose, in the simplest case, the process can be completedwith switched-off vaporizing sources, but otherwise with the sameparameters as in the case of the preceding gradient layer. However, itwas found to be advantageous either to increase the hydrocarbon fractionin the gas flow in the course of the deposition of the pure DLC layer orto lower the noble gas fraction or, particularly preferably, to carryout both measures jointly. Here also, the use of the coils 23 and 25 toform a longitudinal magnetic field, as described above, again has aspecial significance for maintaining a stable plasma.

During the application of the pure DLC layer, after the switching-off ofthe Cr target, the medium frequency supply is adjusted to remainconstant and the argon flow remains the same, the acetylene ramp startedduring the gradient layer is increased for approximately 10 minutesuniformly to a flow between approximately 200 and 400 sccm.Subsequently, for a time period of 5 minutes, the argon flow iscontinuously reduced to a flow between approximately 0 and 100 sccm, forexample to 50 sccm. During the next 55 minutes, the process is completedwhile the settings remain the same. The pressure in the vacuum chamberis typically around 10⁻³ mbar but can be up to an order of magnitudelower or can be somewhat higher than 10⁻³ mbar. The upper coil isoperated with an excitation current of about 10 A and the lower coilwith an excitation current about one third of that of the top coil.

The deposition of the DLC layer thus takes place by a plasma assistedCVD (chemical vapor deposition) process. The plasma assistance comesfrom the plasma generated by the ion source 21 in combination with thevacuum in the chamber and the magnetic field generated in the chamber bythe upper and lower coils 23 and 25 respectively as well as thecontributions to the magnetic field of the other magnets that arepresent or in operation such as the magnets associated with themagnetrons (which are operative to generate magnetic fields even ifmagnetron sputtering is not taking place).

These conditions lead to a relatively high deposition rate and theionization of the plasma is ensured by the presence of the argon gas.The depositing rate will typically be about 1 to 2 microns per hour.

The DLC coating has a hardness of about 25 GPa and a coefficient offriction of about 0.2. It has a hydrogen content of about 13% and aresistance of around 500 kOhm. The adhesion of the DLC coating, whichcan be measured according to DVI 3824, Sheet 4 is very good and can beclassified as HF1 according to the DVI 3824 document.

The layer roughness of the DLC layer has a value of Ra=0.01−0.04; Rz asmeasured according to DIN is <0.8, and usually <0.5.

There are many other possibilities for depositing a DLC layer on a steelworkpiece. For example, some possible processes which can be used in thepresent invention are described in various prior art documents. Thus, aplasma assisted chemical vapor deposition technique for depositingalternating layers of DLC and silicon-DLC layers as a hard coating withgood frictional properties and hardness as well as corrosion resistanceis described in EP-A-651069.

EP-A-600533 describes a method of depositing a DLC coating on an ironsubstrate with a graded transition layer of a-Sil-xCx:H by PACVD usingsilane gas enriched with hydrogen for the Si source and methane enrichedwith hydrogen for the carbon source. A thin layer of Si of 15 nmthickness is first deposited followed by the graded layer of 25 nmthickness, with the proportion of Si decreasing and the proportion of Cincreasing, and is capped by a relatively thick DLC layer to a totallayer thickness of 2.3 microns.

DE-C-19513614 also describes the manufacture of DLC layers on steelsubstrates by a plasma enhanced CVD process operated in a pressure rangebetween 50-1,000 Pa using a thin graded Si carbon layer similar to thatof EP-A-600533. The deposition process uses a bipolar voltage sourceconnected to the workpieces and the bipolar voltage source is designedsuch that during the deposition process the positive pulse duration issmaller than the negative pulse duration. As a result, layers aredeposited in the range of from 10 nm to 10 μm and of a hardness ofbetween 15-40 GPa.

Yet another method for applying hard DLC layers by plasma enhancedchemical vapor deposition is described in U.S. Pat. No. 4,728,529. ThisUS document describes a method for depositing DLC while applying an HFplasma, during which the layer formation takes place in a pressure rangeof between 10⁻³ and 1 mbar using an oxygen-free hydrocarbon plasma whichincludes an admixed noble gas or hydrogen.

DE-A-19826259 describes multilayer structures of metal carbide layers(titanium carbide or chromium carbide) alternating with a-C:H (DLC)layers.

Once a DLC coating of the desired thickness has been achieved the PVDcoating process is complete and the workpieces can be transferred toanother vacuum chamber such as FIG. 5 for the deposition of the ALDcoating.

Turning first to FIGS. 3A to 3C there can be seen a sequence of stepsfor the formation of a first ALD layer. In the step of FIG. 3A aworkpiece or article 12 having an-O—H terminated surface is created.This can be done in a vacuum chamber, described later with reference toFIG. 5, by admitting water to the chamber under CVD (chemical vapordeposition) conditions, especially under PECVD (plasma enhanced chemicalvapor deposition) conditions, i.e. in the presence of a plasma, as iswell known from the field of wafer bonding. Prior to this step thesubstrate can be subjected to extensive cleaning and etching, forexample under PVD (physical vapor deposition) conditions, e.g. bysubjecting the surface to argon ion bombardment as discussed below withreference to FIG. 5.

Once the —O—H terminated surface has been formed the water is removedfrom the chamber by the vacuum pump and the substrate with the —O—Hterminated surface is exposed to an atmosphere of trimethyl aluminium(CH₃)₃Al under the same conditions and this results in an Al atom takingthe place of the hydrogen atom and the other two bonds of the aluminiumatom being respectively occupied by a CH₃ group. This situation is shownin FIG. 3B. The reaction with the trimethyl aluminium has now ceasedbecause no further possibilities for chemical reaction exist. The excesstrimethyl aluminium, together with the CH₄ formed by the reaction oftrimethyl aluminium with the hydrogen atoms of the —O—H terminatedsurface

((CH₃)₃Al+H→(CH₃+H+₂CH₃Al) and CH₃+H→CH₄),

are extracted by the vacuum system and the reaction has stoppedchemically after the generation of just one atomic (molecular) layer.

In the next step water is again admitted to the chamber under CVD orPECVD conditions and leads to the following reaction on the surface withthe CH₃ terminated Al:

2CH₃+H₂O→2CH₄+2(—OH)

The 2-OH radicals bond to the aluminium to result in the situation shownin FIG. 3C. These reactions typically take place in the temperaturerange from 100° C. to 400° C. The CH4 which is formed is sucked away outof the vacuum chamber by the vacuum pump together with the excess watervapor. Again the reaction is chemically stopped once all CH₃ groups havebeen substituted by —OH groups.

It will be appreciated that the situation shown in FIG. 3C equates tothat of FIG. 3A and therefore the process can be repeated each timebuilding up a further ALD layer of Al₂O₃. In principle there is no limitto the number of layers that can be built up in this way although morelayers signifies longer treatment times and therefore no more layersthan are necessary are provided. In the example given of the depositionof Al₂O₃ layers these are high quality layers which are very dense andable to stop corrosive substances reaching the substrate 10. It shouldbe noted that the present invention is not restricted to the depositionof Al₂O₃ layers but can in principle be used with all layer materialscapable of being grown by ALD including: Al₂O₃, TaO, SiO₂, TiO₂, Ta₂O₅,HfO₂, a mixed layer comprising two or more of the foregoing oxides, amultilayer structure comprising alternating layers of two or more of theforegoing oxides or even a DLC layer such as a ta-C layer if it can begrown by ALD. The deposition of these materials by ALD can be done usingthe reagents described in the IC Knowledge publication “2004 ICTechnology”. One particular application for the ALD process is in themanufacture of integrated circuits and the process is described in thisconnection in some detail in the IC Knowledge publication “2004 ICTechnology”. The details described there can be of assistance inrealizing the present teaching and the disclosure of that reference inthis respect is included herein by reference.

A much longer listing of coatings which can be produced by ALD processescan be found in the paper by Riikka L. Puurunen entitled “Surfacechemistry of atomic layer deposition: A case study for thetrimethylaluminum/water process” which issued in the Journal Of AppliedPhysics 97, 121301 in 2005, pages 121301-1 to 121301-52. This papergives extensive details of the ALD-process and summarizes the work whichhas been published by others in the field. The details described therecan be of assistance in realizing the present teaching and thedisclosure of that reference in this respect is included herein byreference. As confirmed by the Puurunen paper referenced above the termALD layer or atomic layer deposition is somewhat misleading. Althoughthe process can be conveniently considered as if each cycle of theprocess is used to deposit one or more layers each essentially one atomthick if the coating used is a coating of an element such as Cu, Mo, Ni,Ta, Ti or W in the above list. If the coating is molecular, e.g. anAl₂O₃ coating, then the name is strictly speaking incorrect butinternationally understood. Moreover, as stressed by Puurunen, theactual growth per cycle of the ALD process can be less than one, sincenot all sites on the substrate or on the preceding ALD layer arenecessarily reactive sites for a variety of reasons.

It should be noted that the ALD process described above is one exampleof how the process can take place and is in no way to be understood as alimiting example. The trimethyl aluminium can also bond “simultaneously”on two OH groups on the surface and then have only one methyl groupsticking out. Both will occur. Which one will occur preferentially hasto do (among others) with the degree of steric hindrance, which more orless means what has the best geometrical fit. Another variant of the ALDprocess using trimethyl aluminium and water is described in the document“Plasma Assisted Atomic Deposition of TiN films, Jun. 23, 2004” authoredby Stephan Heil of the Technical University of Eindhoven. The detailsdescribed there can be of assistance in realizing the present teachingand the disclosure of that reference in this respect is included hereinby reference.

In addition it is known to deposit Al₂O₃ by ALD using oxygen (O₂) as aprecursor. This is particularly attractive because the use of waterrequires significant purging of the ALD chamber. The use of O₂ for thedeposition of is described Al₂O₃ by ALD is for example described in thePhD thesis of Dr. Stephan Heil dated Jun. 29, 2008 and entitled “Atomiclayer deposition of Metal Oxide and Nitrides”. The details describedthere can be of assistance in realizing the present teaching and thedisclosure of that reference in this respect is included herein byreference.

A first coated article 12 made in accordance with the present inventionis shown in FIG. 4A. The article, of which only a surface region 110 isshown here and which is referred to as the substrate in the following,has at least one first layer 112 applied by a PVD (physical vapordeposition) process or by a CVD (chemical vapor deposition) process,e.g. a PECVD (plasma enhanced chemical vapor deposition) process at thesurface region 110 and a second layer 114 comprising one or more atomiclayers of an electrically insulating material deposited by an ALD(atomic layer deposition) process at the same surface region 110.

The PVD or CVD layer 112 is an electrically insulating layer asdescribed above with reference to FIGS. 1 and 2 and generally has a highhardness and high wear resistance, optionally with a low coefficient offriction. In this example the layer 110 is a DLC layer applied directlyonto the surface of a martensitic steel workpiece, i.e. without a bondlayer or a graded layer between the DLC layer and the workpiece,although such layers can be provided if desired or if necessary toobtain good adhesion of the DLC layer and/or to prevent spalling. TheALD layers 114 are Al₂O₃ layers. The DLC layer 112 typically has acolumnar structure and/or a porous structure which would otherwise allowcorrosive substances such as liquids or gases to reach the substrate andcause corrosion there.

Such a columnar structure and/or porous structure would, in the absenceof the electrically insulating ALD layer 114 be a poor coating from thepoint of view of electrical isolation despite the fact that the coatingitself is a good insulator. The point is that the columnar or porousstructure means small areas of the article surface are effectivelyexposed and, particularly with thin coatings, can readily lead to localconducting pathways which must be avoided. This is achieved by theconformal sealing layer in the form of the ALD layer (layer system) 114.

In a bearing component such as a rolling element bearing the coatingwould normally be applied to the outer surfaces of the outer bearingrace and/or to the radially inner surfaces of the inner bearing race,but generally not to the raceway surfaces themselves which are incontact with the rolling elements. An exception to this exists howeverwhen the layer that is applied by a PVD (physical vapor deposition)process, by a CVD (chemical vapor deposition) process, or by a PECVD(plasma enhanced chemical vapor deposition) process (but excluding anALD process or a plasma enhanced ALD process) has a high hardness and ahigh resistance to wear such as a DLC layer because then the raceways ofthe inner an/or outer races could also be coated. This applies even ifthe ALD layer applied to the aforementioned layer wears away from thesurface of the aforementioned layer, since the ALD layer is only verythin.

Coatings applied to the outer surfaces of the outer bearing race and/orto the radially inner surfaces of the inner bearing race do notnecessarily have to be extremely hard or wear resistant because they arenot intended to move within the housing or on the associated shaft.Nevertheless, hard and wear resistant coatings are preferred at suchlocations because the inner and outer races may shuffle within thehousing or on the shaft.

Since the coatings of the present invention tend to be extremely hard orwear resistant and are also extremely thin they can also be formed onthe raceway surfaces if they are sufficiently hard and wear resistant.Theoretically at least the coating could also be provided on the rollingelements if desired.

As shown in FIG. 4A the surface region 110 of the article or substratehas a PVD or CVD layer 112 deposited directly on it and the ALD layer(layer system) 114 is deposited on the PVD or CVD layer 112. Theenlarged view of FIG. 4B is very instructive in connection with thisembodiment. Here the enlarged view has been drawn in exaggerated mannerto show the columnar structure of the PVD or CVD layer 112. Purely forthe sake of illustration FIG. 4B illustrates the interstitial passages116 formed between the columns 118 of the PVD or CVD layer 112.

This embodiment recognizes and exploits an important advantage of theALD process, that the atomic layer growth can take place in deep andnarrow gaps, i.e. here on the side walls of the interstitial passagesand on the side walls of open pores and of any other defects such ascracks. This means that even if only a few layers are grown by ALD theseare sufficient to seal the PVD layer. If enough ALD layers are grownthey can seal the open pores and interstitial passages as shown in FIG.4B and achieve excellent electrical insulation.

However, this is not necessary for the open pores, interstitial spacesor other defects to be fully filled with ALD grown material. It issufficient if the ALD layer 114 only lines the side walls of anycolumns, pores or other defects that are present. This situation isshown in FIG. 4D. Generally speaking it is sufficient if the ALD layer114 comprises a plurality of monolayers having a thickness in the rangefrom 1 nm to 50 nm. Thin layers of 1 nm or a few nm in thickness can bedeposited relatively quickly because the number of repetitions (cycles)of the ALD process necessary to build up the layer 114 is restricted.

When the conformal ALD layer 114, which is itself usually extremelyhard, wears significantly in use then it wears down until the surface ofthe PVD or CVD layer is exposed, as shown in FIGS. 4C and 4E andthereafter wear is insignificant for a long period of time as a resultof the hard DLC layer. During this long period of time the substrate 10is protected by the PVD or CVD layer 112 which remains sealedelectrically by the ALD layer material which lines or fully fills the“openings” in the PVD or CVD layer 112.

The embodiment of FIGS. 4A to 4E is thus also beneficial when the ALDlayer has worn down to the free surface of the PVD or CVD layer because,although the corrosive substances can reach the free surface of the PVDor CVD or PECVD layer they still cannot reach the surface of the articleitself.

Naturally, the PVD layer 112 of FIGS. 4D and 4E (in particular) stillhas a porous structure or a columnar structure with interstitialpassages into which corrosive substances can penetrate. They cannothowever reach the substrate because of the ALD layer (layer system) 114which extends down to the actual surface of the article in suchpassages.

The PVD or CVD layer 112 can also comprise a layer system (not shown butcomprising a plurality of different PVD and/or CVD layers or analternating layer system or a superlattice structure) or a graded layer.Such layer structures are well known per se.

In this embodiment the ALD layer (layer system) 114″ can—withoutrestriction—be one of Al₂O₃, SiO₂, TiO₂, Ta₂O₅, HfO₂ mixed layers of anyof the foregoing and multilayer structures of two or more of theforegoing

The PVD or CVD layer 112, can—without restriction and without includinga possible bond layer—comprise one of a DLC layer, a metal-DLC layer, ora layer of the same composition as the ALD layer. The PVD layer can forexample also be an Al₂O₃ layer.

An apparatus for the deposition of ALD coatings will now be explainedwith reference to FIG. 5.

The treatment chamber 130 shown in FIG. 5 has a central at leastsubstantially rectangular form when viewed from any of its sides. Achamber door not shown in FIG. 5 is pivotally connected to the frontside of the chamber about vertical pivot axles, i.e. axles which lieparallel to the plane of the drawing. The rear side of the chamber couldbe connected by a load lock to a correspondingly designed opening at oneside of the chamber 14 of FIG. 1 which is normally closed by a doorlocated within the opening, that door of the chamber 14 may carry amagnetron and associated cathode. However, in practice such a load lockis not necessary, the treated articles from the chamber of FIGS. 1 and 2can simply be transported in the normal ambient atmosphere to thechamber of FIG. 5. Also one chamber for applying a DLC coating, such asa chamber in accordance with FIGS. 1 and 2, could supply coatedworkpieces to a plurality of apparatuses for applying an ALD coating,such as apparatuses in accordance with FIG. 5.

The arrangement can be such that the workpiece table 20 of FIG. 1together with the workpieces 12 can be to be transferred afterdeposition of the PVD coating 112 into the ALD chamber. If a load locksystem is used this can be done without loss of vacuum and withoutcontamination of the workpiece surface. The table 20 can rotate in thechamber 130 if desired but this is not essential. The transfer system isnot shown but can be designed as in customary load lock systems. Theapparatus could also be designed as a cluster system with a plurality ofsatellite ALD chambers such as 130 arranged around one chamber fordepositing DLC coatings, such as is shown in FIGS. 1 and 2.

When the door and the load lock are closed the chamber 130 is closed onall sides. The door can be opened to permit access to the interior ofthe chamber and removal of the ALD coated workpieces on the table 20.The reference numeral 132 refers to a duct for the connection duct for aperformance vacuum pump (not shown) such as a diffusion pump, a cryopump or simply a mechanical pump which serves in known manner togenerate the necessary vacuum in the treatment chamber. That vacuum maybe of the order of 100 millitorr although it is certainly not necessaryto go so low as there is a thermal space in the chamber of hightemperature. The pressure can generally lie in a range from 1 to 1000millitorr. Disposed opposite the vacuum connection duct 162 is a plasmagenerator 164 for generating a plasma from O₂ gas supplied to thechamber 130 via a valve system (not shown but including a flow regulatorand an on/off valve) through port 166. The reference numeral 168identifies an rf plasma generator basically comprising a coil 170 fedwith a rf energy from source 172.

The reference numeral 174 refers to a source of inert gas such as argonwhich can be admitted to the chamber directly via the valve 176 duringpurging cycles and indirectly via the valve 178 and the container 180when admitting Al(CH₃)₃ to the chamber 130 as a precursor for thedeposition of Al₂O₃ layers by ALD. For this purpose a further valve orvalve system 182 is present between the container 180 and the chamber130 and can be controlled electronically (as can all other valves in theapparatus) to permit a predetermined quantity of Al(CH₃)₃ entrained bythe flow of argon to enter the chamber 130 via port 184.

The plant of FIG. 5 can be operated as follows:

First of all, the atmosphere in the chamber 130 is evacuated via duct162 and replaced by argon. This is done, in known manner, by operationof the vacuum pump and simultaneous supply of argon via valve 176 toflush the originally present residual air from the vacuum chamber 130.Chamber 130 is usually heated to between 200 and 400° C. by wallheaters.

The apparatus is then changed over to the oxygen admission cycle and aplasma is generated in the chamber by the rf-generator and the oxygensupplied thereto. Thereafter a predetermined quantity of Al(CH₃)₃ isadded to the chamber for formation of the first Al₂O₃ layer by ALD.Thereafter the process is repeated until the desired number of ALDlayers have been generated by the plasma enhanced ALD process. Once thefinal layer has been deposited by the ALD process, i.e. once the ALDlayer (layer system) 114 has been completed the articles can now beremoved from the chamber by opening the chamber doors.

It should be noted that the example of a cluster plant for carrying outthe

PVD and/or CVD process and the ALD process as described above is givenpurely by way of example and that the plant can take very differentforms.

Since the ALD layer or layer system 114 is relatively thin and can bedeposited relatively quickly in a time frame comparable to that requiredfor depositing the layer 112 by PVD or CVD a cluster arrangement may notbe the ideal layout.

The complete apparatus could for example be realized as a long tubularplant with successive stations for PVD deposition processes and/or CVDdeposition processes and ALD deposition processes through whichindividual articles move. The entire tubular plant can be evacuated bythe use of load locks for admitting articles to the plant and removingthem from the plant without loss of vacuum. Local gas supplies to theindividual stations and local vacuum pumps can also be provided tomaintain desired atmospheres in the individual stations through whichthe articles sequentially move, for example on a conveyor belt. Such anarrangement can help minimize wastage of the gases required and enhanceuseful process time relative to non-profitable vacuum generationperiods. However, it is probably easiest just to use two separatechambers one for depositing the DLC layer and one for depositing the ALDlayers.

Some examples will now be given of advantageous layer systems which canbe deposited in accordance with the present teaching with reference toFIGS. 6A to 6F.

EXAMPLE 1

Turning first to FIG. 6A there can be seen a surface region 110 of anarticle in the form of a bearing race of 100Cr6 steel which is providedwith a layer 112 of hydrogen free ta-C layer of 4 microns thickness. Thelayer 112 has a structure similar to that illustrated in FIG. 4D. On topof the layer 112 there is a layer 114 of Al₂O₃ of 10 nm thicknessdeposited by ALD using the apparatus of FIG. 5. In this case no bondlayer is provided because the Cr content of the steel and the carbon ofthe ta-C layer are considered adequate.

EXAMPLE 2 (FIG. 6B)

This example is similar to Example 1 but has an a-C:H layer (a DLClayer) deposited as described in detail above on a thin bond layer 112′of chromium with a graded chromium carbide layer deposited using theapparatus of FIG. 1 as described above in detail or by using a chromiumcarbide target (cathode). The bond layer 112′ is only relatively thin,and in this example is 10-300 nm thick.

EXAMPLE 3 (FIG. 6C)

In this example the coating is similar to the coating of Example 2 butincludes an additional layer 112″ of Al₂O₃ of 200 nm-2 μm thicknessdeposited by PVD in the apparatus of FIG. 1. For this purpose theapparatus of FIGS. 1 and 2 is modified so that it can carry out dualmagnetron sputtering from two opposed magnetron cathodes of Aladditionally provided in the vacuum chamber 14 for reactive sputteringusing oxygen in the chamber atmosphere. The reactive sputtering of Al₂O₃is described in detail in the European patent application published asEP-A-2076916. The apparatus can also be designed to make use of thecombined use of HIPIMS and dual magnetron sputtering as described in theEuropean patent application 11007077.8.

EXAMPLE 4 (FIG. 6D)

In this example the layer structure is a two layer structure as shown inFIG. 6A but the layer 112 is an a-C:H layer of 1-4 microns thicknessdeposited by a CVD process (but not an ALD process). The ALD layer 114of Al₂O₃ again has a thickness of 10 nm.

EXAMPLE 5 (FIG. 6E)

This example is the same as Example 2 but the ALD layer 114 has agreater thickness of 26-50 nm.

EXAMPLE 6 (FIG. 6F)

This example is similar to Example 1 but here the layer 112 is a layerof Al₂O₃ 4 microns thick deposited by PVD (dual magnetron sputtering ina reactive oxygen atmosphere) as in Example 3.

EXAMPLE 7

In this example the layer structure resembles that of Example 6 (FIG.6E) but with a bond layer 112′ of Cr or Ti deposited by PVD. It shouldbe noted that it is unimportant whether the bond layer is electricallyconducting or insulating since even if it is electrically conductivethis situation is no worse than the component itself being electricallyconductive.

In all examples the layers have the same thicknesses as in otherexamples unless something different is specifically stated. Thus thelayer 112 is generally 1-4 microns thick. The layer 112″, if present, isgenerally 50 nm-2.5 μm thick and the ALD layer 114 is generally 10-50 nmthick.

In all examples the thickness of the layers are 10nm for the layer 112′,4 microns for the layer 112, 10 nm for the layer 112″ and 10 nm for thelayer 114 unless otherwise stated. The electrical insulation andcorrosion resistance of all examples was found to be good.

1. A bearing component of steel (12), the bearing component 5 having atleast one layer (112; 112, 112; 112, 112; 112″) having a high currentinsulation property applied by at least one of a PVD (physical vapordeposition) process, a CVD (chemical vapor deposition) process, and aPECVD (plasma enhanced chemical vapor deposition) process (but excludingan ALD process or a plasma enhanced ALD process) at at least one surfaceregion (110) of said component (12), said at least one layer (112; 112,112; 112, 112; 112″) comprising a non-conductive oxide layer selectedfrom the group of materials comprising an Al₂O₃ layer, a Ta_(x)O_(y)layer, an Si_(x)O_(y) layer, a mixed layer comprising two or more of theforegoing oxides, a multilayer structure comprising alternating layersof two or more of the foregoing oxides and a DLC layer such as a ta-Clayer, there being at least one ALD layer (114) comprising at least onelayer of a material deposited by an ALD (atomic layer deposition)process on said at least one layer having a high hardness and a highcurrent insulation property, the ALD layer itself having a high currentinsulation property and comprising a material or layer structureselected from the said group of materials.
 2. A bearing component inaccordance with claim 1, the bearing component being selected from thegroup comprising a component of a linear bearing, a component of arolling element bearing and a component of a sliding bearing.
 3. Abearing component in accordance with claim 1, the component beingselected from the group comprising a bearing race, a rolling element atapered roller, a barrel roller, a needle roller, a bearing ball and arolling element cage.
 4. A bearing component in accordance with claim 1,wherein the at least one layer applied by at least one of a PVD(physical vapor deposition) process, a CVD (chemical vapor deposition)process, and a PECVD (plasma enhanced chemical vapor deposition) process(but excluding an ALD process or a plasma enhanced ALD process) has acomposition and that composition is at least substantially the same as acomposition of the layer deposited by the ALD process.
 5. A bearingcomponent in accordance with claim 1, wherein the said at least onelayer applied by at least one of a PVD (physical vapor deposition)process, a CVD (chemical vapor deposition) process, and a PECVD (plasmaenhanced chemical vapor deposition) process (but excluding an ALDprocess or a plasma enhanced ALD process) has a thickness in the rangefrom 0.5 microns to 4microns and the ALD layer has a thickness in therange from 1 nm to 1000 nm.
 6. A bearing component in accordance withclaim 5, wherein the thickness of the ALD layer is in the range from 10to 50 nm.
 7. A bearing component in accordance with claim 1, wherein thesteel of which the article is made is a martensitic grade of steel.
 8. Abearing component in accordance with claim 7, wherein the martensiticgrade of steel is at least one of a bearing steel and a coldworkablesteel.
 9. A bearing component in accordance with claim 7 wherein themartensitic grade of steel is one of 100Cr6, 100CrMn6, 16MnCr5, C80 orX30CrMoN 15 1, and a steel in accordance with Din: 1.4108 or SAE:AMS5898.
 10. A bearing component in accordance with claim 1, wherein theat least one layer having a high hardness and a high current insulationproperty applied by at least one of a PVD (physical vapor deposition)process, a CVD (chemical vapor deposition) process, and a PECVD (plasmaenhanced chemical vapor deposition) process (but excluding an ALDprocess or a plasma enhanced ALD process) is provided with an additionalPVD layer deposited by one of arc sputterin,), magnetron sputtering,reactive magnetron sputtering and dual magnetron sputtering, any of theforegoing optionally with plasma enhancement prior to the deposition ofthe ALD layer.
 11. A bearing component in accordance with claim 10,wherein the arc sputtering is carried out by filtered arc sputteringusing at least one rectangular or round sources and the laser arcdeposition is carried out using at least one cylindrical source and themagnetron sputtering is carried out using at least one of a rectangularsources, a round source, a cylindrical source and first and secondopposed magnetron sources.
 12. A bearing comprising a bearing componentof steel (12), the bearing component having at least one layer (112;112, 112; 112, 112; 112″) having a high current insulation propertyapplied by at least one of a PVD (physical vapor deposition) process, aCVD (chemical vapor deposition) process, and a PECVD (plasma enhancedchemical vapor deposition) process (but excluding an ALD process or aplasma enhanced ALD process) at at least one surface region (110) ofsaid component (12), said at least one layer (112; 112, 112; 112, 112;112″) comprising a non-conductive oxide layer selected from the group ofmaterials comprising an Al2O3 layer, a TaxOy layer, an SixOy layer, amixed layer comprising two or more of the foregoing oxides, a multilayerstructure comprising alternating layers of two or more of the foregoingoxides and a DLC layer such as a ta-C layer, there being at least oneALD layer (114) comprising at least one layer of a material deposited byan ALD (atomic layer deposition) process on said at least one layerhaving a high hardness and a high current insulation property, the ALDlayer itself having a high current insulation property and comprising amaterial or layer structure selected from the said group of materials.13. A machine comprising a bearing component of steel (12), the bearingcomponent having at least one layer (112; 112, 112; 112, 112; 112″)having a high current insulation property applied by at least one of aPVD (physical vapor deposition) process, a CVD (chemical vapordeposition) process, and a PECVD (plasma enhanced chemical vapordeposition) process (but excluding an ALD process or a plasma enhancedALD process) at at least one surface region (110) of said component(12), said at least one layer (112; 112, 112; 112, 112; 112″) comprisinga non-conductive oxide layer selected from the group of materialscomprising an Al₂O₃ layer, a Ta_(x)O_(y) layer, an Si_(x)O_(y) layer, amixed layer comprising two or more of the foregoing oxides, a multilayerstructure comprising alternating layers of two or more of the foregoingoxides and a DLC layer such as a ta-C layer, there being at least oneALD layer (114) comprising at least one layer of a material deposited byan ALD (atomic layer deposition) process on said at least one layerhaving a high hardness and a high current insulation property, the ALDlayer itself having a high current insulation property and comprising amaterial or layer structure selected from the said group of materials.